The present invention relates to a diffraction grating lens for focusing a laser beam or the like to a minimum spot, and relates more particularly to a diffraction grating lens suitable for an objective lens of an optical disk recording/reproducing apparatus and an optical recording/reproducing apparatus using the diffraction grating lens.
Recently, a high-density information recording has been required for a video disk, a read-only optical disk such as a CD, and a recordable and reproducible optical disk of additional recording type or of rewritable type. For this purpose, it has become necessary to provide a light source and an optical system which can be applied for an optical disk recording/reproducing apparatus and which can focus a laser beam for recording and reproducing to a minimum spot.
In general, a diffraction limit spot diameter s of a light focused by a lens having a numerical aperture NA for a light source having a wavelength .lambda. is given by the following expression. EQU s=c.multidot..lambda./NA (1)
In this case, c takes a value of 0.67 when a diameter is selected to take the intensity of 1/e.sup.2, for example, in the Gaussian distribution. Since the numerical aperture of the lens in general is not larger than 1 even as a maximum, s can not be made smaller than c.lambda. as is clear from the above expression.
An effective method of obtaining a minimum spot is to shorten the wavelength .lambda. of the light source. However, when a compact semiconductor laser is used as a light source, there is a limit to the shortening of the wavelength. Further, when the wavelength becomes shorter than the ultraviolet zone, the conventional optical systems can not be used from the limit of the transparent zone of lens materials.
As a method for exceeding the limit shown by the above-described expression (1), a solid immersion lens (SIL) has been proposed (such as, for example, by B. D. Terris et al,: Appl. Phys. Lett., 65 (1994) p. 388). This SIL is designed to focus an incident light beam in this lens.
When an incident light beam is focused in the SIL, the focusing spot diameter can be made smaller than that obtained when incident light beam is focused in the air. This is because when an incident light beam is focused in a medium of which refractive index n is larger than 1, the numerical aperture NA can be made larger than 1.
In an optical system using an SIL, a part of the light beam emitted from this SIL becomes what is called an evanescent light-wave. The evanescent light-wave is a light-wave which exists as a localized wave without propagational property. Since this evanescent light-wave is attenuated rapidly, an object such as an optical disk must be kept in the vicinity of the light emitting plane of the SIL in order to utilize the minimum spot obtained by the SIL.
However, an apparatus of this type has the following problems. First, since the SIL has a large curvature, a lens of non-aberration is necessary, of which preparation is extremely difficult. Second, since the lens becomes necessarily thick, it is difficult to make this lens compact or match position with other parts.
As a lens which utilizes the evanescent beam, there is one as disclosed in the Japanese Patent Application KOKAI Publication No. 7-93797.
According to the technique disclosed in this publication, a grating lens is used as a mean for forming a focusing spot. This grating lens utilizes a diffraction grating group formed in an equivalent distance concentric shape of the distance shorter than that of a light wavelength and generates an evanescent light-wave near the center of this diffraction grating group.
When this grating lens is used as an objective lens of an optical disk recording and reproducing apparatus, a plane on which the diffraction grating group is formed is faced with the surface of an optical disk in a state of being almost in contact with the surface of this optical disk. In other words, unlike the SIL, this grating lens does not generate an evanescent light beam by focusing an incident light beam in the lens but generates an evanescent light beam within the diffraction grating.
However, according to the above-described grating lens, since it should be used by keeping the surface on which the diffraction grating is formed to be almost in contact with the optical disk, there arises a problem in the protection of the diffraction grating pattern. Moreover, since the evanescent light-waves are generated by the whole of the diffraction grating according to this grating lens, there is a possibility that these light-waves reduce the SIN ratio. Further, there is another problem that the optical spot diameter does not necessarily become the so-called minimum spot diameter.